The plasma transferred wire arc thermal spray process melts a feedstock material, usually in the form of a metal wire or rod, by using a constricted plasma arc to melt the tip of the wire or rod, removing the molten material with a high-velocity jet of ionized gas issuing from a constricting orifice. The ionized gas is a plasma and hence the name of the process. Plasma arcs operate typically at temperatures of 18,000-25,000.degree. F. (10,000.degree.-14,000.degree. C.). An arc plasma is a gas which has been heated by an electric arc to at least a partially ionized condition, enabling it to conduct an electric current. A plasma exists in any electric arc but the term plasma arc is associated with plasma generators which utilize a constricted arc. One of the features which distinguishes plasma arc devices from other types of arc generators is that, for a given electrical current and gas flow rate, the arc voltage is significantly higher in the constricted arc device. In addition, a constricted arc device is one which causes all of the gas flow with its added energy to be directed through the constricted orifice resulting in very high exiting gas velocities, generally in the supersonic range. There are two modes of operation of constricted plasma torches. One of these modes, known as the "non-transferred" mode. Characteristically, the non-transferred plasma torch has a cathode and an anode in the form of a nozzle. In general, practical considerations make it desirable to keep the plasma arc within the nozzle with the arc terminating on the inner nozzle wall. However, under certain operating conditions, it is possible to cause the arc to extend outside the nozzle bore and then fold back, establishing a terminal point for the arc on the outside face of the anode constricting nozzle. The other mode of plasma operation is termed "transferred-arc" mode. In this mode of plasma operation, the plasma arc column extends from the cathode through a constricting nozzle and then leaves the torch and is terminated on a workpiece anode which is located electrically spaced and isolated from the plasma torch assembly.
In the plasma transferred wire arc thermal spray process, the plasma arc is constricted by passing it through an orifice downstream of the cathode electrode. As plasma gas passes through the arc, it is heated to a very high temperature, expands and is accelerated as it passes through the constricting orifice often achieving supersonic velocity on exiting the orifice, towards the tip of the wire feedstock. Typically, the different plasma gases used for the plasma transferred wire arc thermal spray process are air, nitrogen or a admixture of argon and hydrogen. The intensity and velocity of the plasma is determined by several variables including the type of gas, its pressure, the flow pattern, the electric current, the size and shape of the orifice and the distance from the cathode to the wire feedstock.
The prior art plasma transferred wire arc process shown in FIG. 1 is a schematic representation of the apparatus disclosed in our U.S. Pat. No. 5,296,667. The process operates on direct current from a constant current type power supply 17. The cathode electrode 11 is connected to the negative terminal of the power supply 17 through a high frequency generator 21 which is employed to initiate an electrical arc between the cathode 11 and the constricting nozzle 10. The high frequency arc initiating circuit is completed by the momentary closure of the pilot-arc relay contact 18 allowing direct current to flow from the positive terminal of power supply 17 through pilot resistor 19 to the constricting nozzle 10, through the high frequency arc formed between the cathode 11 and the constricting nozzle 10, through the high frequency generator 21 to the negative terminal of the power supply 17. The high frequency circuit is completed through the bypass capacitor 20. This action heats the plasma gas which flows through the orifice 13. The orifice 13 directs the heated plasma stream from the cathode electrode 11 towards the tip of the wire feedstock 14 which is connected to the positive terminal of the power supply 17. The plasma arc attaches to or "transfers" to the wire tip and is thus referred to as a transferred arc. The wire feedstock 14 is advanced forward by means of the wire feed rolls 16a and 16b, which are driven by a motor which is not shown. When the arc melts the tip of the wire, the high-velocity plasma jet impinges on the wire tip and carries away the molten metal, simultaneously atomizing the melted metal into fine particles and accelerating the thus formed molten particles to form a high-velocity spray stream entraining the fine molten particles.
In order to initiate the transferred plasma arc a pilot arc must be established. A pilot arc is an arc between the cathode electrode 11 and the constricting nozzle 10. This arc is sometimes referred to as a non-transferred arc because it does not transfer or attach to the wire feedstock as compared to the transferred arc which does. A pilot arc provides an electrically conductive path between the cathode electrode 11 within the plasma transferred wire arc torch and the tip of the wire feedstock 14 so that the main plasma transferred arc current can be initiated. The most common technique for starting the pilot arc is to strike a high frequency or a high voltage direct voltage (d.c.) spark between the cathode electrode 11 and the constricting nozzle 10. A pilot arc is established across the resulting ionized path generating a plasma plume. When the plasma plume of the pilot arc touches the wire tip, an electrically conductive path from the cathode electrode to the anode wire tip is established. The constricted transferred plasma arc will follow this path to the wire tip.
In the practical use of the plasma transferred wire arc thermal spray process for the spraying of electrically conductive wires or rods, there are several problems which are encountered. One of these problems is a condition known as "double arcing". Double arcing can occur as a result of one of several causes such as malfunction of the wire feed system, a kink in the wire or inadvertently shutting off the wire feeder. The result of the phenomena of double arcing can be serious damage to the constricting nozzle as well other components used to guide the wire into the plasma transferred arc. The problem of double arcing has been dealt with in one manner as described in our U.S. Pat. Nos. 5,296,667 and 5,442,153.
Another problem exists with the practical use of the plasma transferred wire arc process occurs due to the electrical potential difference that exists between the wire and the constricting nozzle. As a result of this potential difference, metal dust is attracted to the face of the constricting nozzle. As this metallic dust builds up, various conditions of electrical shorting between the constricting nozzle and the anode wire can occur. This electrical shorting creates major damage to the plasma transferred wire arc torch components. In addition, a major problem has been found when any form of wire feed hesitation occurs. Wire hesitation can be caused by any one of a number of reasons all dealing with either the wire surface condition (i.e kinks in the wire or other surface irregularities) or malfunctioning of the wire feed mechanism or wire feeder. The result of wire feed hesitation is the tendency of the transferred-arc to burn back along the wire, resulting in severe damage to wire guiding and support components of the plasma transferred wire arc gun head. As can be seen in FIG. 1, it is necessary to provide both electrical contact to the wire 14 as well as providing proximate close alignment of the wire to the centerline of the plasma jet stream in order to assure uniform melting of the wire tip as it advances into the plasma transferred arc.
Additionally, in operation of the prior art apparatus and method of coating bores, build-up of coating material can form on the outer surface of the torch which faces the surface being coated. This occurs especially in coating of small bores because a small part of the molten particles which are being propelled to the substrate do not adhere to the substrate but bounce back onto the torch surface. Since, in the operation of the prior art, it is necessary to maintain a 90.degree. geometric relationship between the pilot nozzle face and the axis of the wire in order to prevent secondary arcing, the hot molten particles will bounce directly back to the plasma torch assembly since the angle of incidence is 90.degree. and therefore the angle of reflection will also be generally 90.degree.. This build-up of metal particles can be detrimental to the proper performance of the plasma transferred wire arc torch since it can cause electrical shorting of various component of the torch as well as interfering with the proper flow of gases from the various orifices of the pilot nozzle.
In addition, problems can occur during the starting of the spraying process which causes a "spit" or large molten globule to be formed and propelled to the substrate and included into the coating as the coating builds up on the substrate. This problem occurs due to the variable time delay of between 50 and 100 milliseconds that can occur between the time the transferred-arc power supply and the wire feeder are energized and the time the transferred-arc is fully established. Based on a typical wire feed rate of 160 inches per minute, and a variable time delay period of 50 milliseconds, more than 1/8th inch of wire can move past the arc point before the transferred-arc starts melting the wire. This extension of 1/8th inch of wire is what can form into the "spit" or globule. Since there is a variable period of 50 milliseconds, the problem cannot be solved by simply controlling the starting point of the wire feeder or establishing a predetermined acceleration rate of the wire feed rate. Similarly, globules may be propelled if the transferred arc is lost due to a power failure or power reduction allowing the wire feed rate to continue without adequate melting.
Accordingly, it is an object of the present invention to provide an improved plasma transferred wire arc thermal spray apparatus and method which solves the above described problems.
A further object of the present invention is to provide a plasma transferred wire arc thermal spray method and apparatus in which an extended plasma arc jet is created and which is employed as a electrical contacting means to a metal wire as well as acting to atomize and propel molten metal particles to a substrate to form a dense coating while eliminating the arc burn back along the wire.
Another object of the present invention is to provide an improved plasma transferred wire arc thermal spray apparatus and method in which an extended plasma arc jet is created and which is employed as a electrical contacting means to a metal wire as well as acting to atomize and propel molten metal particles to a substrate to form a high density coating while avoiding the formation of a "spit" at the onset of the spraying process.
Yet another object of the present invention is to provide an improved plasma transferred wire arc thermal spray apparatus and method which eliminates metal dust attraction to the face of the constricting nozzle.
Still another object of the present invention is to provide an improved plasma transferred wire arc thermal spray method and apparatus which eliminates secondary arcing between a wire feed and the constricting nozzle.
It is another object of this invention to provide an improved plasma transferred wire arc thermal spray method and apparatus which eliminate the occurrence of build-up of metal particles on the outer surface of the torch assembly.
A further object of the present invention is to provide a apparatus and method for producing high performance, well bonded coatings which are substantially uniform in composition and have a very high density with very low undesirable oxide content (oxides which are other than the lowest molecular oxide form producing the lowest dynamic coefficient of friction) formed within the coating.
A further object of the present invention is to provide an improved plasma transferred wire arc apparatus and method which is simple in construction and may be operated at relatively low gas consumption and is relatively maintenance-free.
It is a further object of the present invention to provide an improved plasma transferred wire arc thermal spray method and apparatus which can be utilized to apply a thermal spray coating to the inside of cylinder bores such as automobile engine cylinder bores, by rotating the plasma transferred wire arc torch around the axis of the wire.